Behind casing water volume flow rate measurement using concentric gamma ray detectors

A method is described for measuring the location, linear flow rate and volume flow rate of undesired water flow behind well casing in a producing well. An instrument is lowered into the borehole Inside the instrument is a source of fast neutrons which causes the nuclear reaction O16(n,p)N16, and at least two gamma-ray detectors longitudinally spaced from the source and each other. At least one of the detectors comprises an inner cylindrical detector and an outer cylindrical shell detector coaxially aligned. The borehole environs are irradiated with fast neutrons, and the detectors sense gamma rays resulting from the decay of the unstable isotope N16. Count signals from the inner and outer detectors are separated into separate signals and a ratio signal is generated. Count signals from the longitudinally-spaced detectors are combined to derive an indication of the linear flow rate of undesired behind-casing water flow in a preferred direction. An indication of the radial distance from the inner detector to the undesired water flow is generated from the ratio signal. Combining the linear flow rate indication and the radial distance indication provides an indication of the volume flow rate of the undesired behind-casing flow. (LL)